Switched rhombic automatic direction finding antenna system and apparatus

ABSTRACT

An antenna system which includes a four-element structure composed of inductances wound on high Q ferrite material and arranged in the form of a rhombus in combination with switching and tuning circuits provides both the bi-phase and constant phase signals required for automatic direction finding (ADF) equipment, thus eliminating the sense antenna requirement in these systems.

United States Patent Layton 3,671,970 1 June 20, 1972 ..nm e m n w a y em .h m e mu b ca .m HG L 69 J 46 99 W .11 N .m 1 m 7 2 m 54 w m I A n 23P [54] SWITCHED RHOMBIC AUTOMATIC DIRECTION FINDING ANTENNA SYSTEM ANDAPPARATUS Attorney-Glenn Orlob, Kenneth W. Thomas and Conrad O. .Gardner[72] Inventor: John E. Layton, Seattle, Wash.

[73] Assignee: The Boeing Company, Seattle, Wash.

[22] Filed: Aug. 31, 1970 [21] Appl. No.: 68,300

[57] ABSTRACT An antenna system which includes a four-element structurecomposed of inductances wound on high Q ferrite material and arranged inthe form of-a rhombus in combination with switching and tuning circuitsprovides both the bi-phase and constant phase signals required forautomatic direction finding (ADF) equipment, thus eliminating the senseantenna requirement in these systems.

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8 Claims, 5 Drawing Figures References Cited UNITED STATES PATENTSPATENTEDmzo I972 3.671.970

SHEET 10F 4 A F C'O/YDTROL //UD/0 00w? UN/ T Y says/5 5E/Y5E ANTENNAANTENNA COUPLER A D F BEAR/N6 RECEIVER IND/6470A auAmA/vm 5555 m0?ANTENNA COEBECTOI? 95:91. PRIOR ART ADF CONTROL AUD/0 OUTPUT UNIT 105ES'W/TCH 4 ADP R BEAR/N6 7 RECEIVE /ND/CATOR RF SIGNALS INVENTOR.

JOHN E. [/IYTON PATENTEDJUHZO I972 3.671.970

INVENTOR. JOHN E LA YTON A TTOFNE Y SWI'ICIIED RHOMBIC AUTOMATICDIRECTION FINDING ANTENNA SYSTEM AND APPARATUS This invention relates toradio automatic direction finding apparatus and more particularly toantenna arrangements and circuits for tuning and switching these antennaarrangements to satisfy automatic direction finding functions withoutthe utilization of separate sense antennas in these apparatus.

Conventional ADF systems require a sense antenna to produce a constantphase signal which combined with the figure of eigh bi-phase loopantenna signal provides the means for resolving the ambiguity inherentin bearings obtained using only a loop antenna.

Typical sense antennas offer little or no discrimination toelectrostatic interference and, as a consequence, fields produced by theaccumulation of precipitation static charges on the antenna and airframeas well as corona discharge are coupled into the ADF receiveressentially unattenuated. The seriousness of this problem can beappreciated when it is recognized that sense antennas on many largeaircraft are formed by depositing metallic film on contoured fiberglaspanels which are already installed for aerodynamic reasons. The area ofthese panels can be quite large, ranging on actual production typeairplanes from 2,500 to 6,000 square inches. Precipitation static chargebuild-up on panels of this size together with normal atmospheric noiseexperienced in the LF/MF (low frequency/medium frequency) spectrum maymake ADF systems or apparatus unusable.

In contrast to the flush mounted loop antennas currently utilized in theart, the sense antenna is required to be designed for each new airframesince it is usually an integral part of a fairing or other skin surfacewhich is necessary for aerodynamic or structural reasons. Because of thedifliculties arising from problems associated with each new airframe,the location of the sense antenna often must be compromised, andinferior system performance can result when the sense antennaenvironment becomes noisy. Service range becomes sacrificed and bearingaccuracy is degraded.

An attempt to solve the preceding problems afl'ecting system performanceintroduced by the sense antenna by attempting to derive the sense signalfrom the outputs of conventional loop antennas would appear to result infailure since the loop antenna patterns do not have the characteristicswhich lend themselves to such an approach, and, further, their effectivecapture area is extremely small. In addition, the composite loop signalis at or near zero levels because of the ADF receiver system'sgoniometer shaft position when the bearing is achieved. At this point,little or no signal would be available for use as a constant phasereference.

Because of these difficulties, it is an object of the present inventionto provide an ADF system which eliminates prior art sense antennashortcomings.

It is another object of this invention to provide an antenna system ofnovel design which provides the functions now accomplished by the stateof the art loop and sense antenna combination.

It is yet another object of this invention to provide a switched rhombicantenna for use in ADF system installations on aircraft, boats, mobileland vehicles, and fixed direction finding stations.

It is yet a further object of this invention to provide means for tuningand lobe-switching a rhombic antenna for use at ADF band frequencies.

It is a further object of this invention to provide a rhombic antennahaving large inductance elements would on high permeability material forachieving high effective circuit in the antenna.

It is yet another object of this invention to provide a compact tunedrhombic antenna having dimensions that are a small fraction of thewavelength to which the rhombic is tuned.

These and other objects of the invention are achieved in a switching andtuning arrangement which couples to an ADF receiving system a rhombicantenna having large inductance elements and operated at resonance. Theinvention itself, and

additional objects and advantages thereof. will best be understood fromthe following description when read in conjunction with the accompanyingdrawings in which:

FIG. 1 is a block diagram of a conventional ADF system;

FIG. 2 is a block diagram of an ADF system embodying the principles ofthe invention;

FIG. 3 is a schematic diagram of a series tuned rhombic antenna in anADF system in accordance with an-embodiment of the present invention;

FIG. 4 is a schematic diagram of a parallel tuned rhombic antenna in anADF system in accordance with a further embodiment of the presentinvention;

FIG. 5 is a graph illustrative of Q and bandwidth characteristics of theparallel tuned antenna configuration of FIG. 3 helpful in understandingthe nature of the results achieved by this compact rhombic antenna.

Turning now to FIG. 1 of the drawings, there is shown a known ADF systemwherein the system components are identified and the signal processingrequired to satisfy the direction finding function is also indicated.Referring now to FIG. 2, there is shown an ADF system according to thepresent invention which does not require a separate sense antenna forits operation, yet satisfies all of the requirements specified for ADPsystems. In the ADF system embodiments of FIGS. 3 and 4 there is shownan antenna structure of novel design which, in addition to providing allof the functions now accomplished by the conventional loop and senseantenna combination of FIG. 1, eliminates those deteriorations inperformance introduced by the sense antenna hereinbefore mentioned andoccupies substantially the same space required merely by the former loopantenna alone.

The series tuned rhombic antenna of FIG. 3, and the parallel tunedrhombic antenna of FIG. 4 will be seen to comprise conductors l0, l2,l4, and 16 extending in different directions to form the side legs of arhombus. Each of conductors 10, l2, l4, and 16 which formed thefour-element array in the em bodiment test comprised an approximateZOO-turn winding of No. 39/44 Litz wire wound on high Q materialconsisting of ferrite strips 18 manufactured by Indiana General Corp. of

Keasbey, N. J., termed Q1 material having a dimension of 3.75 inches by0.725 inches by 0.125 inches and a specified permeability of 400, with aCurie Point of 662? F. which makes the antenna structure also compatiblewith elevated skin temperatures of supersonic aircraft. The four-elementarray forms a structure which is mounted on the surface of the aircraft,e.g., the belly or top of the aircraft, in a depression in the metallicskin thereof. Utilizing the parallel tuned rhombic antenna configurationof FIG. 4, in which the four elements comprising high inductancewindings l0, l2, l4, and 16 wound on high permeability cores 18comprised four type No. 2003 700-microhenry miniature antenna structuresmanufactured by the Miller Company of Los Angeles, Calif. satisfactoryresponse and operation in all three of the present ADF frequency rangeswas achieved. Measurements of the effective height of two commerciallyavailable ADF antennas in terms of transverse magnetic field componentwas found to be 1.4 millimeters and 0.75 millimeters respectively,whereas the present high Q tuned rhombics effective height was found tobe 34.5 millimeters. This measurement while made only at a frequency of300 KHz, however, is indicative of what may be expected of the presentantenna of the higher frequencies in the ADF bands.

An exact appreciation of the importance of the present ADF antenna maybe had by reference to the frequency response curve of the paralleltuned rhombic antenna shown in FIG. 5 which indicates a Q of and abandwidth of 3 KHz. Antenna gain measurements made using a state of theart ADF fixed type antenna having the same dimensions as the rhombic asa reference showed the gain of the tuned rhombic resonated at the testfrequency of 300 KHz to be 25 db. Utilization of known full size rhombicantennas at ADF frequencies would, of course, be prohibitive in airbornesystems although at Ul-IF or Vl-IF frequencies where a full size rhombicat 200 to 400 MHz is not geometrically large as, for example, in U. S.Pat. No. 2,968,035 to Sirons where free space wavelength varies between0.75 and 1.5 meters, such antennas have been utilized. The highpermeability of cores l8 and the large inductance of the windings 10,12, 14, and 16 wound thereon makes possible in combination with tuningnetworks, the achievement of very high efi'ective circuit Q in theantenna at the lower frequencies in the present three ADF bands havingfrequency ranges 190 to 400 KHZ, 400 to 840 KHZ, and 840 to 1,750 KI-Iz.Two resulting benefits of significanee include the tuned antennaprovision of additional and heretofore unachieved additional selectivityin ADF systems; and an improved signal plus noise to noise figurerealization as noted previously in the 3 Hz indicated bandwidth.

lncreased range, better discrimination against interfering adjacentchannel signals and drastic reduction in electrostatic interference inthe ADF system result from improved gain and bandwidth properties of thepresent antenna.

Series tuning of the 4-element rhombic array of FIG. 3 to resonance isachieved by the tuning network 20 which tunes and connects in seriesconductors 10 and 14, and 12 and 16. A lobe switching network 22 couplesthe tuned rhombic antenna to ADF receiver 24. Lobe switching network 22periodically interchanges points 26 and 28 as the feed and terminatingpoints of the antenna, thereby providing the overlapping patterns whichyield the signal voltages applied to the stator terminals 30 and 32 ofdifierential capacitor 34. The resulting signal voltages which areapplied to stator terminals 30 and 32 differ in amplitude and phase dueto the shape of the patterns and the phase reversal which takes placewith lobing. These signals provide signals at rotor terminal 36 whichwhen processed in ADF receiver 24 develop servo control through shaft 38for controlled movement of differential capacitor rotor 40. The largerof the two signal voltages present at stator terminals 30 and 32 causesthe ADF receiver 24 servo motor to drive rotor 40 in a direction whichresults in equal amplitude levels. This action differs in an importantrespect from that which occurs in the prior art ADF receiver where theservo motor drives the goniometer into a null. In the present ADFsystem, on either side of the equi-signal shaft position a similar phasereversal occurs which causes the servo to drive the shaft 38 back intothe desired equi-signal position. This shaft 38 position is alsotransmitted to RMI (Radio Magnetic Indicator) 42 as bearing information.

A theoretical study made of bearing errors and assuming perfect cardioidantenna patterns indicated that because of the shape of the cardioidnear the null, bearing errors become excessive at approximately plus orminus 60 degrees from the degree 180 degree heading. It is thereforeapparent that the perfect cardioid pattern is not the desired pattern ifbearing errors arising from pattern shape are to be minimized. As aconsequence, the present ADF system utilizing differential capacitor 34for coupling the signal voltages derived from lobe switching to thereceiver affords the use of variation in capacitor plate shapeconfigurations in addition to modification of antenna geometry toachieve minimum system errors.

Turning now to FIG. 3 and, more specifically, to tuning network 20 inwhich a tuning voltage from ADF receiver 24 is applied to terminals 44and 46 having the polarity indicated, the tuning voltage having anamplitude which varies with the frequency to which the receiver is tunedis supplied by means of resistors 48 to varactor diodes 50 whosecapacitances vary in a manner to cause series resonance to occursimultaneously and be maintained with changes in frequency in each sideof the rhombic between points 26 and 28; viz., in the series circuitpath between circuit points 26 and 28 of the system which includes onone side the series connection of a first inductance means comprisingconductor spirally wound about core 18, capacitor 52, varactor diode 50,capacitor 52, and a second inductance means comprising conductor 14spirally wound about core 18, and on the other side between circuitpoints 26 and 28 which includes conductors 12 and 16. This series tunedconfiguration is preferable over the FIG. 4 parallel tuned configurationwhere tuning is desired over a limited range of frequencies, e.g., oneADF band only. Turning now to lobe switching network 22, it will benoted that the switching voltage is obtained from ADF receiver 24, andthis switching voltage, sometimes termed lobe switching voltage by thoseskilled in the art, when connected in the present ADF system causes eachof circuit points 26 and 28 to become alternately the feedpointrespectively to stator terminals 30 and 32 while the other point isterminated to ground through a terminating resistor 60. Lobe switchingis thus achieved at the frequency of the lobe switching signal, normallyat an audio rate.

Referring now to FIG. 4 showing tuning network 20 and lobe switchingnetwork 22 for the parallel tuned rhombic embodiment, it will be notedthat this balanced configuration requires about twice as many componentsin lobe switching network 22 as was required in the lobing network 22 ofthe series tuned rhombic configuration of FIG. 3. Lobing is achieved inthe switching network 22' by application of the switching voltageobtained from ADF receiver 24 between circuit points 70-71 and 72-73,thereby alternately forward and reverse biasing the diodes connected tocircuit points 70 and 72 and between 71 and 73. During the switchinghalf cycle which forward biases the diodes connected to the Z/2 shuntresistors connected to circuit point 70, the end of the antennacomprising terminals and 81 of conductors 10 and 12 is terminated in theimpedance Z, and the RF signal developed in the parallel tuned rhombicof FIG. 4 is coupled to stator terminal 91 of the differential capacitorvia the forward biased diodes connected to the output transformerprimary in the circuit path at the other end of the antenna comprisingterminals 82 and 83 of conductors 16 and 14. When the switching voltagewaveform reverses polarity, the situation is reversed, i.e., endterminals 80 and 81 now deliver RF developed in the rhombic antenna todifferential capacitor stator terminal 92, and the antenna becomesterminated at end terminals 82 and 83 in the impedance Z formed by thetwo irnpedances Z/2 connected together at circuit point 73. The receivedsignals are then coupled to rotor 40 of the differential capacitor andthen to the signal amplifier and phase detector of the ADF receiver 24where phase detection takes place, resulting in servo drive signals forthe differential capacitor rotor 40 and RMI bearing indicator instrumentin the same manner as indicated hereinbefore in the discussion of theFIG. 3 ADF system signal processing.

Proceeding now to the following description of tuning network 20' inwhich the four elements of the rhombic formed by spiral wound conductors10, 12, 14, and 16 each comprised 1,400 rnicrohenry inductances, only adescription of that portion of tuning network 20 used to tune the upperside of the rhombic loop between terminals 80 and 83 will be detailed,since that portion of tuning network 20 in the lower portion of block20' connected between points 81 and 82 is identical in mode of operationhowever tuning the lower half or other side of the rhombic. The upperhalf of the antenna comprising series-connected elements 10 and 14 istuned by that portion of tuning network 20' coupled between terminals 80and 83 which varies the bias voltage on varactor diode 1 10 from ADFreceiver 24 and via terminals 118 and 119. Terminals 112 and 114 areopen (no connection) for the to 400 KHZ frequency range, terminal 1 12is made positive for tuning over the 400 to 840 KHz frequency range, andterminal 114 is made positive when the 840 to 1,750 Kl-Iz frequencyrange is tuned. These control voltages are made available by the controlhead of the ADF receiver as each ADF band is selected and a controlvoltage for selecting each band of operation is applied to only onediode at a time. Terminal 116 provides a connection to the negativereturn lead from the control head band switch.

In the exemplary tuning network 20 of FIG. 4 according to the invention,components and electrical values are as follows:

Resistors:

R 201 0.1 megohms non R 202 0.1 megohms R 203 0.1 megohms R 204 1.0megohms R 205 1.0 megohms Inductors:

L 301 820 microhenries L 302 152 microhenries Capacitors:

C 401 0.1 microfarads C 402 0.1 microfarads C 403 0.1 microfarads C 4040.1 microfarads C 405 0.1 microfarads Despite the somewhat greatercomplexity of the circuitry for the parallel-tuned embodiment shown inFIG. 4, this embodiment can be used throughout the entire range of ADFfrequency bands, switching of the elements into the tuning network beingaccomplished by conventional band switches currently in use, or by thetuning logic of next generation ADF receivers. In contrast, the seriestuned configuration of FIG. 3 would be preferable for tuning. over asomewhat limited frequency range, e.g., not encompassing the three ADFbands. With currently available voltage variable capacitors utilized inthe present design of FIG. 3, a 2 to 1 frequency range is feasible, 190KHZ to 400 KHZ being tuned with a control voltage variation of 1.25volts to 5.5 volts which is well within the linear portion of thevariable voltage capacitors characteristic curve.

Proceeding now to a functional description of FIG. 3, in thisconfiguration of the antenna, the series tuned version, both halves ofthe rhombic comprised of elements and 14 on one side and 12 and 16 onthe other are simultaneously maintained at series resonance withvaractor diodes 50 and capacitors 52, the latter providing isolationfrom the DC tuning voltage for elements 10, 14, 12 and 16. The reactanceof capacitors 52 is made small with respect to diode capacitors 50 toinsure that the tuning function is provided solely by diode capacitors50. Resistors 48 are made large to confine the RF signals to the tunedantenna elements and to isolate the signal circuits from bias circuitry.

The two halves of the rhombic antenna comprised on one side byferrite-cored inductances 10 and 14 in series with capacitors 50 and 52,and on the other side by ferrite-cored inductances l2 and 16 in serieswith capacitors 50 and 52 are joined at points 26 and 28 which arealternatively connected to terminating resistors 60 and to differentialcapacitor plates 30 and 32 via coupling transformers. Terminatingresistors 60 cause the antenna radiation pattern to assume the requiredcardioid shape which reverses as the lobing signal voltage changespolarity. The lobing voltage insures that when antenna terminal 26 isterminated by resistor 60 via a forward biased diode, the opposite endof the antenna 28 is connected to differential capacitor plate 32 via acoupling transformer by a forward biased diode. Conversely, when thelobing voltage reverses, antenna terminal 28 is connected to terminatingresistor 60 by a forward biased diode, and antenna terminal 26 is nowconnected to differential capacitor plate 30 by a forward biased diode.In the first instance, signals are prevented from reaching differentialcapacitor 34 by a diode being reversebiased by the lobing voltage andterminating resistor 60 for antenna terminal 28 is isolated by areverse-biased diode. In the second instance, signals are prevented fromreaching differential capacitor 34 from antenna terminal 28 by a reversebias on a diode and the terminating resistor 60 for antenna terminal 26is isolated by a reverse bias on a diode.

Signals reaching the receiver 24 via rotor plate 40 of differentialcapacitor 34 will differ in amplitude and phase by virtue of the levelchanges caused by the overlapping cardioid antenna patterns, and willappear as a modulated waveform, the levels changing at the lobingfrequency. The same lobing signal is applied to the phase detector inthe receiver where the differingamplitude and phase develop controlvoltage for the servo motor causing it to turn differential capacitor 34shaft 38 in such a direction as to equalize the two signal levelscoupled to rotor 40 from fixed plates 30 and 32. Drive signals from thephase detector will cease when this position of the rotor shaft isreached and the Radio Magnetic Indicator (RMI) 42 whose drive shaft iscoupled to the servo motor will indicate the magnetic bearing relativeto aircraft heading from which the signal is being received.

Turning now to a functional description of FIG. 4, it will be observedthat this configuration of the antenna is the parallel tuned version andis comprised basically of series connected elements 10 and 14 on oneside of the array which are in paral lel with series connectedcapacitors C-40l, C-402, C-403 and varactor diode 110. On the other sideof the array series connected elements 12 and 16 are similarly parallelconnected with identical capacitors and varactor diode 110. Reactance ofcapacitors C-40l, C-402 and C403 is made negligible with respect to thatof varactor diode 110 to insure its effectiveness as the principaltuning element. Capacitors C-402 and C-403 isolate the tuned circuitfrom the DC tuning voltages on varactor diode 110. Likewise resistorsR-204 and R-205 are made large to confine the RF signals to the tunedcircuits in each half of the antenna. Diodes 110 are matched and aresimultaneously biased via resistors R-204 and R-205 so as to maintainparallel resonance in both halves of the antenna. Tuning voltage isapplied at terminals 118 and 119. Lobing voltage obtained from thereceiver 24 alternately forward and reverse bias diodes associated withthe terminating resistors at points 70 and 73, and the couplingtransformers 100 in the following manner. When points 70 and 71 arepositive with respect to points 72 and 73-the right end of the antenna,i.e., points 82 and 83, becomes terminated by resistors Z/2 via theassociated forward biased diodes. Likewise, the left end of the antenna,i.e., points and 81, becomes terminated by the coupling transformer viathe forward biased diodes, and the RF signal is coupled to the receiver24 via terminal 92 and rotor 40'of differential capacitor 34. Reversebiased diodes associated with terminating resistors Z/2 in the left handnetwork isolate them from the circuit, and the reverse biased diodesassociated with coupling transformer 100 in the right hand networkisolate it from differential capacitor 34. With reversal of the lobingvoltage, right hand end of the antenna, i.e., points 82 and 83, iscoupled to transformer 100 by forward biased diodes, and the left end ofthe antenna, i.e., points 80 and 81 is terminated by resistors 2/2 atpoint 72 by forward biased diodes. Left hand coupling transformer 100 isisolated from the circuit by reverse biased diodes and the terminatingresistors Z/2 at point 73 are isolated from the associated network byreverse biased diodes. RF signals are thereby coupled via right handtransformer 100 to receiver 24 via terminal 91 and rotor 40 ofdifferential capacitor 34.

The lobing voltage accomplishes the alternate terminating and receivingfunctions of the antenna as described previously for the series tunedversion shown in FIG. 3.

In order to tune the antenna throughout the complete ADF band offrequencies, i.e., from 190 KHZ through 1,750 KI-lz, inductances L-301and L-302 are caused to be in parallel with series elements 10 and 14and varactor diode 1 10 on one side of the antenna and with serieselements 12 and 16 and th e varactor diode associated with that side ofthe antenna by forward biasing the diode via R-201, or the other diodevia R-202. With neither L-301 nor L-302 in the circuit, i.e., diodesreverse biased, the bias voltage range for varactor diode tunes eachhalf of the antenna to resonance in the frequency range KHz to 400 KHz.With a diode forward biased, inductor L-301 is placed in parallel withelements 10 and 14 on one side and similar inductor L-301 on the otherside of the antenna is placed in parallel with elements 12 and 16 by aforward biased diode. The resulting inductance in each half of theantenna is thereby reduced to such a value that the tuning voltage rangefor varactor diode 110 now resonates each half of the antenna in thefrequency range of 400 KHz to 840 KHZ. With diodes associated withinductances 13-301 reverse biased, and diodes forward biased viaresistors R-202, inductances L-302 are placed in parallel with theirassociated antenna halves and the varactor diode 110 tuning voltageresonates the antenna in the frequency range of 840 KHz to 1,750 KHZ.Diodes are isolated from the switching voltage source by individualresistors R-201 and R-202 as well as the common return resistor R-203,all of which are made large so as to confine signal voltages to thetuned circuits. Capacitors (2-401 and C-402 prevent the switchingvoltages for diodes from influencing the tuning varactor diodes 110while providing a complete path for the RF currents in the tunedcircuits. Capacitors C-401 are made large for this reason.

Reference to FIG. 5 shows a realized Q and bandwidth of 100 and 3 KB:respectively. While this data has not been taken on the series ofconfiguration, the response of the series configuration during screenroom tests indicates it is equal to or superior to the parallelconfiguration in respect to these parameters. Varactor tuning in thedisclosed embodiments proved effective and tracking using I. T. T. typeBA-l63 type diodes in the circuits shown operated satisfactorily thoughmatched pairs were utilized.

In FIG. 5, besides the previously noted Q of nearly 100 at 300 KHz, itshould also be observed that the gain with respect to the half-powerpoints 501 and 502 is slightly over 28 db.

While in the preceding description of embodiments of the inventionexemplary circuit means and connections as well as specific electricalvalues have been set forth by way of example, it will be evident tothose skilled in the art that in the light of the present disclosurevariations may be made without departing from the true spirit and scopeof the invention and without exercise of more than ordinary skill in theart. Accordingly, it is not desired that the invention be restricted tothe exemplary details, other than is required by the recitations of theappended claims.

I claim:

1. A rhombic antenna array having feed and terminating points for use ina selected frequency band comprising:

first, second, third, and fourth elements having dimensions that are asmall fraction of the smallest wavelength in the selected frequencyband,

said first, second, third, and fourth elements each comprising helicallywound conductor members on ferrite cores,

tuning network means coupled to said first, second, third,

and fourth elements for tuning said antenna to a substantially resonantcondition between said feed and terminating points,

said tuning network means comprising a first varactor diode and firstand second capacitors connected in series between said first and secondelements.

2. A rhombic antenna array having feed and terminating points for use ina selected frequency band comprising:

first, second, third, and fourth elements having dimensions that are asmall fraction of the smallest wavelength in the selected frequencyband,

said first, second, third, and fourth elements each comprising helicallywound conductor members on ferrite cores,

tuning network means coupled to said first, second, third,

and fourth elements for tuning said antenna to a substantially resonantcondition between said feed and terminating points,

said tuning network means comprising a first varactor diode and firstand second capacitors connected in series between said first and secondelements,

said tuning network means further comprising a second varactor diode andthird and fourth capacitors connected in series between said third andfourth elements.

3. in an automatic direction finding system, the combination of:

an automatic direction finder receiver,

a four element rhombic antenna having feed and terminating points, eachof said elements comprising helically wound conductor members on ferritecores,

tuning network means coupled to said rhombic antenna,

lobe switching network means operating to periodically interchange thefeed and terminating points of the antenna,

differential capacitor means coupled between said lobe switching meansand said automatic direction finding receiver, said diflerentialcapacitor having first and second stator 5 plates coupled respectivelyto said feed and terminating points of said four element rhombicantenna, and a rotor plate,

said automatic direction finder receiver comprising signal processingmeans including signal amplifier and phase detector means coupled tosaid rotor plate for developing servo control for controlled movement ofsaid differential capacitor rotor.

4. A rhombic antenna array having feed and terminating points forcoupling to a variable frequency receiver providing a tuning voltagehaving an amplitude which varies with the frequency to which thereceiver is tuned comprising:

first, second, third, and fourth elements having dimensions that are asmall fraction of the smallest wavelength in the selected frequencyband,

said first, second, third, and fourth elements each comprising helicallywound conductor members on ferrite cores,

tuning network means coupled to said first, second, third,

and fourth elements for tuning said antenna to a substantially resonantcondition between said feed and terminating points,

said tuning network means comprising variable voltage capacitor meansresponsive to said tuning voltage for achieving and maintaining saidsubstantially resonant condition with changes in frequency of saidreceiver.

5. In an automatic direction finding system, the combination of:

a four element rhombic antenna having feed and terminating points, eachof said elements comprising helically wound conductor members on ferritecores,

tuning network means coupled to each of said four elements for tuningsaid antenna to a substantially resonant condition between said feed andterminating points,

lobe switching network means operating to periodically interchange thefeed and terminating points of the antenna,

an automatic direction finder receiver and means including a servodriven capacitor rotor shaft coupled between said lobe switching networkmeans and said automatic direction finding receiver for providingbearing information dependent upon said shaft position.

6. In an automatic direction finding system, the combination of:

a four element rhombic antenna for providing a cardioid type pattern andhaving feed and terminating points, each of said elements comprisinghelically wound conductor members on ferrite cores,

tuning network means coupled to each of said four elements for tuningsaid antenna to a substantially resonant condition between said feed andterminating points,

lobe switching network means operating to periodically interchange thefeed and terminating points of the antenna,

an automatic direction finder receiver, and

difi'erential capacitor means for coupling signal voltages derived fromlobe switching to said receiver.

7, In combination:

a four element rhombic antenna having feed and terminating points, eachof said elements comprising helically wound conductor members on ferritecores,

lobe switching network means operating to periodically interchange thefeed and terminating points of the antenna,

an automatic direction finder receiver comprising signal amplifier andphase detector means, and

differential capacitor means comprising first and second statorterminals and a rotor, said lobe switching means coupling the feed pointto said first or second stator terminals, and said rotor coupled to saidsignal amplifier and phase detector.

8. An automatic direction finder receiver antenna for 75 mounting on thesurface of an aircraft comprising:

1. A rhombic antenna array having feed and terminating points for use ina selected frequency band comprising: first, second, third, and fourthelements having dimensions that are a small fraction of the smallestwavelength in the selected frequency band, said first, second, third,and fourth elements each comprising helically wound conductor members onferrite cores, tuning network means coupled to said first, second,third, and fourth elements for tuning said antenna to a substantiallyresonant condition between said feed and terminating points, said tuningnetwork means comprising a first varactor diode and first and secondcapacitors connected in series between said first and second elements.2. A rhombic antenna array having feed and terminating points for use ina selected frequency band comprising: first, second, third, and fourthelements having dimensions that are a small fraction of the smallestwavelength in the selected frequency band, said first, second, third,and fourth elements each comprising helically wound conductor members onferrite cores, tuning network means coupled to said first, second,third, and fourth elements for tuning said antenna to a substantiallyresonant condition between said feed and terminating points, said tuningnetwork means comprising a first varactor diode and first and secondcapacitors connected in series between said first and second elements,said tuning network means further comprising a second varactor diode andthird and fourth capacitors connected in series between said third andfourth elements.
 3. In an automatic direction finding system, thecombination of: an automatic direction finder receiver, a four elementrhombic antenna having feed and terminating points, each of saidelements comprising helically wound conductor members on ferrite cores,tuning network means coupled to said rhombic antenna, lobe switchingnetwork means operating to periodically interchange the feed andterminating points of the antenna, differential capacitor means coupledbetween said lobe switching means and said automatic direction findingreceiver, said differential capacitor having first and second statorplates coupled respectively to said feed and terminating points of saidfour element rhombic antenna, and a rotor plate, said automaticdirection finder receiver comprising signal processing means includingsignal amplifier and phase detector means coupled to said rotor platefor developing servo control for controlled movement of saiddifferential capacitor rotor.
 4. A rhombic antenna array having feed andterminating points for coupling to a variable frequency receiverproviding a tuning voltage having an amplitude which varies with thefrequency to which the receiver is tuned comprising: first, second,third, and fourth elements having dimensions that are a small fractionof the smallest wavelength in the selected frequency band, said first,second, third, and fourth elements each comprising helically woundconductor members on ferrite cores, tuning network means coupled to saidfirst, second, third, And fourth elements for tuning said antenna to asubstantially resonant condition between said feed and terminatingpoints, said tuning network means comprising variable voltage capacitormeans responsive to said tuning voltage for achieving and maintainingsaid substantially resonant condition with changes in frequency of saidreceiver.
 5. In an automatic direction finding system, the combinationof: a four element rhombic antenna having feed and terminating points,each of said elements comprising helically wound conductor members onferrite cores, tuning network means coupled to each of said fourelements for tuning said antenna to a substantially resonant conditionbetween said feed and terminating points, lobe switching network meansoperating to periodically interchange the feed and terminating points ofthe antenna, an automatic direction finder receiver and means includinga servo driven capacitor rotor shaft coupled between said lobe switchingnetwork means and said automatic direction finding receiver forproviding bearing information dependent upon said shaft position.
 6. Inan automatic direction finding system, the combination of: a fourelement rhombic antenna for providing a cardioid type pattern and havingfeed and terminating points, each of said elements comprising helicallywound conductor members on ferrite cores, tuning network means coupledto each of said four elements for tuning said antenna to a substantiallyresonant condition between said feed and terminating points, lobeswitching network means operating to periodically interchange the feedand terminating points of the antenna, an automatic direction finderreceiver, and differential capacitor means for coupling signal voltagesderived from lobe switching to said receiver.
 7. In combination: a fourelement rhombic antenna having feed and terminating points, each of saidelements comprising helically wound conductor members on ferrite cores,lobe switching network means operating to periodically interchange thefeed and terminating points of the antenna, an automatic directionfinder receiver comprising signal amplifier and phase detector means,and differential capacitor means comprising first and second statorterminals and a rotor, said lobe switching means coupling the feed pointto said first or second stator terminals, and said rotor coupled to saidsignal amplifier and phase detector.
 8. An automatic direction finderreceiver antenna for mounting on the surface of an aircraft comprising:a four element rhombic antenna array, each of said elements comprisinghelically wound conductor members on ferrite cores, voltage controlledtuning means including first capacitor means for tuning two of saidelements to resonance and second capacitor means for tuning theremaining two of said elements to resonance.